Olympic skeleton simulation

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2010

A video-based measuring technique helps racers minimize aerodynamic drag.

KEY CONCEPTS
• In the sport known as skeleton, racers travel down a track on a sled at speeds up to 110 kilometers per hour.
• Among the factors that racers need to minimize are the frictional force occurring as the steel runner on the sled meets the ice and aerodynamic drag.
• A simulator has been developed that uses a technique called Digital Particle Image Velocimetry to help racers reduce aerodynamic drag.

At this year’s Winter Olympics in Vancouver, Canada, many of you may have seen athletes moving down a track on a sled at speeds up to 110 kilometers per hour in a sport known as skeleton. Time is of the essence in this sport as racers are separated from each other by hundredths of a second.

There are a number of factors that can be controlled to enhance the ability of an athlete to move down the track at the fastest possible speed. Timothy Wei, professor of engineering in the mechanical, aerospace and nuclear engineering department at Rensselaer Polytechnic Institute in Troy, N.Y., says, “An athlete is propelled by gravity down the track once they push off at the start. Factors such as aerodynamic drag, minimization of friction while sledding on the ice and finding the shortest line down the track are the most important that need to be mastered by the athlete to be successful.”

In a previous TLT article, a number of variables were examined that impact the friction of ice (1). Among the issues are the ice surface itself, the temperature of the ice, the load, the polarity of the ice, impurities, molecular adhesion and shearing.

These variables are usually uniform for all of the racers except for the load. Wei says, “The governing organization for skeleton (FIBT) mandates that the runners used are prepared from the same alloy of steel and by the same machine shop in Europe. No foreign substances can be used, and the runners also cannot be heated prior to use.” Wei indicates these rules are very similar to how Formula One race cars are regulated by that sport’s governing body.

Sleds are inspected at the beginning of the season and are marked with an electronic stamp. Prior to each race, the temperature of the runners is measured and must not exceed the temperature of a reference runner by 4 C.

Wei says, “The frictional force occurring as the steel runner meets the ice is pretty much evened out by the rules. This happens to be a larger overall factor than the aerodynamic penalty that can hinder the racer’s speed.”

But minimization of aerodynamic drag is still a valuable asset that could enable a racer to gain valuable hundredths of seconds on his or her competitors. A strategy has now been developed to help skeleton racers in this area.

DIGITAL PARTICLE IMAGE VELOCIMETRY
Wei and his associates reproduced a section of a skeleton track behind a wind tunnel that is currently used at RPI. He says, “We built a straightaway section of the track that is 12 feet long-by-4 feet wide-by-2.5 feet wide. The floor of the track is raised above the wind tunnel floor to basically allow the setup to be exposed to clean air. The skeleton sled sits on four little pads, which are situated below the floor and contain load cells.”

Hooked up to the load cells are sensors that measure the pitch, roll and balance of the sled. In other words, the setup will measure how much the skeleton athlete will turn the sled left, right, forward and backward. Figure 3 shows an athlete lying on a sled situated in the simulator.


Figure 3. A technique known as Digital Particle Image Velocimetry helps skeleton racers improve their ability to reduce aerodynamic drag. (Courtesy of Rensselaer Polytechnic Institute)

Wei says, “Four of the sensors are set up to measure the downward force on the athlete and two sensors measure the drag force encountered. These sensors indicate how well-centered and how well the athlete’s weight is distributed on the sled.”

Wei indicates that the normal force, which is a measurement of the impact of gravity on the athlete, is a significant factor. A heavier athlete is more likely to go faster down the track than a lighter one.

A window was also cut in the bottom of the test track and a computer monitor installed to enable the athlete to see changes in aerodynamic drag in real-time. A clear sidewall was also built so that coaches can observe the tests.

Athletes from the U.S. Olympic Skeleton Team took advantage of this setup to reduce aerodynamic drag before the Vancouver Olympics. The athlete lies down on the sled and is subjected to a steady stream of air from the wind tunnel exhaust. Air blows at the athlete at a speed of nearly 100 kilometers per hour.

Wei indicated that sessions last up to 30 minutes. He says, “It usually takes about five to 10 minutes to do the evaluation. In one case, we found that an athlete had raised her foot by up to 20 to 30 degrees. When she lowered the foot, aerodynamic drag decreased by 15% to 20%.”

Wei used a video-based flow measurement technique known as Digital Particle Image Velocimetry. He says, “A thin sheet of light is created from a green pulse laser that is shined over the shoulders of the athlete. Theatrical fog is then introduced, and we videotaped its movement against the bodies and heads of the athletes to identify ways to reduce drag.”

Use of this procedure has proven to be valuable in helping the athlete perform better. Wei said that the skeleton sled is built very low to the ice with the athlete just inches above the ice. He adds, “The athlete’s head gets rattled during the high frequency acceleration down the track, which typically lasts about one minute. Athletes will only make three runs per day because the stress and g forces encountered leave them exhausted. This means that use of our simulator is invaluable in providing assistance to the athlete.” 

Wei also used this technology to assist swimmers on the U.S. Swim Team for several years. As competition at the highest level becomes more intense, athletes are looking for any advantage over their competitors. Minimization of factors such as friction and aerodynamic drag are important and will continue to be so in the future.

Further information on the high-tech simulator can be found here or by contacting Wei at weit@rpi.edu. 

REFERENCE
1. Nalence, N. and Rostro, B. (2006), “Cold Play,” TLT, 62 (12), pp. 22–31.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.